1. Field of the Invention
The present invention relates to an optical modulator and an optical modulation device.
The present application claims priority on Japanese Patent Application No. 2014-67114, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
Optical communication devices operating with wavelengths ranging from 1,310 nm to 1,550 nm have been used for local area networks (LANs) and optical fibers used for household appliances. It is preferable to employ silicon-base optical communication devices in which optical function devices and electronic circuits can be integrated on silicon platforms by way of CMOS technologies.
Silicon-base optical communication devices have been developed and applied to waveguides, optical couplers, wavelength filters, optical modulators, etc. Among them, optical modulators serving as active devices have attracted attention among engineers. Additionally, it is generally known that Mach-Zehnder interferometers can be applied to optical modulators using changes of refractive indexes. Optical modulators using Mach-Zehnder interferometers are designed to produce optical intensity modulation signals by way of interference on differences of optical phases in arms including two optical waveguides.
Various types of optical devices and optical modulators have been developed and disclosed in various documents. Patent Literature Document 1 discloses a soliton pulse generating device using a Y-junction Mach-Zehnder interferometer. Patent Literature Document 2 discloses an electro-optic SSB optical modulator having a period domain inverting structure using a Mach-Zehnder interferometer waveguide. Patent Literature Document 3 discloses a semiconductor Mach-Zehnder optical modulator. Patent Literature Document 4 discloses a high-speed silicon-base electro-optic modulator. Patent Literature Document 5 discloses an optical device including an optical demultiplexer, a Mach-Zehnder optical modulator, and an optical multiplexer. Patent Literature Document 6 discloses an optical modulator.
FIG. 11 is a schematic illustration showing an example of an optical modulator using a Mach-Zehnder interferometer. The optical modulator includes a first arm A1 and a second arm A2, which are connected to an optical branch structure A3 and an optical coupling structure A4. The optical branch structure A3 is branched into the arms A1 and A2 in the light-input side while the optical coupling structure A4 couples the arms A1 and A2 together in the light-output side. Light input to the optical branch structure A3 is changed in phase while being guided along the arms A1 and A2. Then, optical signals transmitted through the arms A1 and A2 are combined together via the optical coupling structure A4. Both the arms A1 and A2 are silicon-base electro-optic elements which operate based on voltages so that light is changed in phase due to an electro-optic effect or a thermo-optic effect.
Both the arms A1 and A2 have the same length. Without any voltages, no phase differences occur between the arms A1 and A2 so as to superimpose optical signals having the same wavelength, thus maximizing the intensity of light output from the optical coupling structure A4. With a phase difference π occurring between the arms A1 and A2, optical signals transmitted through the arms A1 and A2 are cancelled out when combined together via the optical coupling structure A4, thus minimizing the intensity of light output from the optical coupling structure A4.
Generally speaking, it is possible to maximize an extinction ratio of light by setting an operating point to the intensity of light output from an optical modulator applied with an intermediate voltage between the maximum voltage maximizing the intensity of light and the minimum voltage minimizing the intensity of light. Any one of arms is set to an initial state applied with a voltage causing an optical phase difference corresponding to a half wavelength, and then an operating point (or a reference point) is set to the intensity of light in the initial state. An optical modulator operates based on an operating point so as to output an optical signal.
An optical phase difference occurs between two arms when a voltage is applied to at least one of two arms. For example, two arms are configured of silicon-base electro-optic elements in which refractive indexes are changed in optical waveguides due to an electro-optic effect or a thermo-optic effect upon applied voltages. Changes of refractive indexes in optical waveguides may cause changes of optical waveguide conductions, thus causing an optical phase difference between two arms having the same length.
A voltage causing an optical phase difference may occur due to an operating voltage of a power source applied to electrode pads of an optical modulator. Herein, a power source is connected to electrode pads via a circuit substrate mounting electrode pads so that electrode pads of a circuit substrate can join electrode pads of an optical modulator.
It is preferable to further develop optical modulators which are reduced in size and which can operate based on a low operating voltage. Patent Literature Document 3 teaches an optical modulator which can operate based on a low operating voltage by dividing electrode pads. Patent Literature Document 4 teaches a silicon-base electro-optic modulator in which a dielectric layer is interposed between a first conductive semiconductor layer and a second conductive semiconductor layer, achieving a PIN diode structure. This aims to improve the response speed while reducing the size of an optical modulator.
As described above, it is possible to achieve an optical modulator operating based on a low operating voltage by dividing an electrode pad into a plurality of electrode pads or by reducing the size of an optical modulator. However, these techniques require that electrode pads of an optical modulator be attached to electrode pads of a drive circuit at a high precision. For this reason, it is impossible to achieve a highly-integrated optical modulator.
Specifically, an optical modulator of Patent Literature Document 3 is designed to juxtapose electrode pads in parallel, and therefore it is necessary to set an electrode interval between adjacent electrodes to be larger than a bonding error. This makes it difficult to realize a highly-integrated optical modulator. Additionally, a silicon-base electro-optic modulator of Patent Literature Document 4 includes a very thin area of about 10 nm causing dynamic changes of a carrier density. This technology needs an optical phase modulation length on the order of millimeters, which makes it difficult to reduce the size of an optical modulator. Since this technology inevitably increases the size of an optical modulator, it is impossible to achieve a highly-integrated optical modulator.